An organic light emitting display is a film light emitting device made of organic semi-conductive material and driven by a direct current voltage, and the film light emitting device includes a glass substrate and a very thin layer of organic material coated on the glass substrate. When current flows through the organic material, the organic material emits lights actively without any backlight.
Because the luminescence brightness emitted by the organic light emitting display is related to a magnitude of the current flowing through the organic light emitting display, the electrical performance of thin film transistors (TFTs) acting as drivers for the organic light emitting display directly influence the display effect of the organic light emitting display. Specifically, a drift in the threshold voltage of a thin film transistor may cause an uneven brightness of the whole organic light emitting display.
To improve the display effect of the organic light emitting display, a driving circuit for pixel compensation is utilized in the organic light emitting display. FIG. 1 is a schematic view of a pixel compensation circuit for an organic light emitting display in the prior art. As shown in FIG. 1, the pixel compensation circuit includes one capacitor and five thin film transistors, among which thin film transistors T2 and T4 are turned on or off under the control of a signal SELECT, and thin film transistors T3 and T5 are turned on or off under the control of a signal EMIT. A reference voltage Vref is inputted via the thin film transistor T3, a data voltage Vdata is inputted via the thin film transistor T2, and a power supply voltage Vdd is inputted via a thin film transistor T1.
During a driving process of the pixel compensation circuit, initially the signal SELECT is at a low level while the signal EMIT is at a high level, such that data DATA is inputted to one end of the capacitor C1 and a threshold voltage Vth of the thin film transistor T1 is detected at the other end of the capacitor C1, and thus voltages on both ends of the capacitor C1 are Vdd−Vth and Vdata, respectively. Then, the signal SELECT changes to a high level and the signal EMIT changes to a low level, therefore the potential at a point B is Vref and the potential at a point A is Vref−Vdata+Vdd−Vth because of a coupling effect of the capacitor C1.
Then, a driving current for the light emitting of the organic light emitting element OLED in FIG. 1 is:Ids=K(Vsg−Vth)2=K(Vdd−(Vref−Vdata+Vdd−Vth)−Vth)2=K(Vdata−Vref)2  (1),where K is a constant. At this time, the magnitude of the driving current of the organic light emitting element OLED is irrelevant to the threshold voltage of the driving transistor, such that a function of pixel compensation is realized.
However, the above mentioned calculation is theoretically ideal. In practice, voltages at both ends of the capacitor C1 change simultaneously when the signal SELECT is at a low level and the signal EMIT is at a high level. If the size of the data DATA in the current frame is much larger than that of the data DATA in the preceding frame, then due to the coupling effect of the capacitor C1 at the moment when the signal SELECT is changed from a high level to a low level, the potential at the point A is pulled up to a very high level instantly. As a result, in the period of detecting the threshold voltage of the thin film transistor T1, the detected threshold voltage Vth′ is inaccurate and is different from the actual threshold voltage Vth by ΔVth, which leads to the inaccuracy of subsequent threshold compensation. That is, if the potential at the point A is Vref−Vdata+Vdd−Vth′, then the driving current of the organic light emitting element OLED isIds=K(Vsg−Vth)2=K(Vdata−Vref+ΔVth)2  (2)
It can be seen from the above equation the pixel compensation is ineffective because of the presence of the ΔVth, and the organic light emitting display still has the problem of uneven brightness.